a. Field of the Invention
This invention relates to a method for determining the geothermal gradient of subterranean formations. More particularly, the invention relates to such a method useful in drilling wells through such subterranean formations.
B. Description of the Prior Art
In the drilling of wells, the temperature or geothermal gradient of the subterranean formations being drilled are important considerations. Different drilling apparatus, procedures, and fluids are required for drilling formations of different temperatures. Generally, the temperature increases in some manner with increasing depth, but the rate of change varies. It would be particularly desirable to have a method for predicting the temperature of formations before drilling into them. A knowledge of the temperature of underlying formations can give an indication to those skilled in the art of the depth at which desirable hydrocarbons are most likely to be encountered. Heretofore, determining the geothermal gradient of a formation prior to drilling into it has not been done. During the drilling operation, the bottom hole temperature can be directly measured by stopping drilling and lowering a thermometer into the hole. From the temperature and depth information, the geothermal gradient may be calculated. However, such methods are time consuming and tend to be inaccurate due to the cooling effect on the wellbore of the drilling fluid being circulated unless the drilling operations are shut in for a thermal equilibrium with the surrounding formation. Shutting in a well during drilling is highly undesirable because it unduly lengthens the expensive drilling operation.
It is an object of this invention to provide a method for determining the geothermal gradient of a subterranean formation.
It is another object to provide such a method which can be used to predict such geothermal gradient below a borehole being drilled into the subterranean formation.
It is still another object to provide such a method which can be used to predict such geothermal gradient without drilling into the subterranean formation.
It is a further object to provide an indirect method of predicting the geothermal gradient of a subterranean formation during drilling of a well, which method is unaffected by the actual temperature of the borehole. Other objects, advantages, and features will become apparent from the following description, drawings, and appended claims.